ABR Type 3 LSA Filtering
This feature module describes filtering interarea routes on an Area Border Router (ABR) with the Open
Shortest Path First (OSPF) protocol and includes the following sections:
•
Feature Overview, page 1
•
Benefits, page 1
•
Restrictions, page 1
•
Related Features and Technologies, page 2
•
Supported Platforms, page 2
•
Supported Standards, MIBs, and RFCs, page 2
•
Configuration Tasks, page 2
•
Configuration Examples, page 4
•
Command Reference, page 5
Feature Overview
The ABR Type 3 link-state advertisement (LSA) Filtering feature extends the capability of an ABR that
is running the OSPF protocol to filter type 3 LSAs between different OSPF areas. This feature allows
only specified prefixes to be sent from one area to another area and restricts all other prefixes. This type
of area filtering can be applied out of a specific OSPF area, into a specific OSPF area, or into and out
of the same OSPF areas at the same time. This feature is supported by the addition of the area filter-list
command.
Benefits
The OSPF ABR Type 3 LSA Filtering feature gives the user improved control of route distribution
between OSPF areas.
Restrictions
Only type-3 LSAs that originate from an ABR are filtered.
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Supported Platforms
Related Features and Technologies
The OSPF ABR Type 3 LSA Filtering feature is an extension of the OSPF routing protocol. For more
information about configuring OSPF and configuring route summarization and filtering, refer to the
“OSPF” chapter of the Cisco IOS Release 12.0 Network Protocols Configuration Guide, Part 1 and
Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1.
Supported Platforms
The OSPF ABR Type 3 LSA Filtering feature is supported by the following platforms in Cisco IOS
Release 12.0(15)S:
Note
•
Cisco 7200 series
•
Cisco 7500 series
•
Cisco 12000 series
Support for the Cisco 10000 and 10700 was added in Cisco IOS Release 12.0(17)ST.
Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To obtain lists of MIBs supported by platform and Cisco IOS release, and to download MIB modules,
go to the Cisco MIB web site on Cisco Connection Online (CCO) at
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
RFCs
No new or modified standards are supported by this feature.
Configuration Tasks
See the following sections for configuration tasks for the OSPF ABR Type 3 LSA Filtering feature.
Each task in the list is identified as either optional or required:
•
Configuring ABR Type 3 LSA Filtering (required)
•
Verifying ABR Type 3 LSA Filtering (optional)
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Configuration Tasks
Configuring ABR Type 3 LSA Filtering
To filter interarea routes into a specified area, use the following commands beginning in router
configuration mode:
Command
Purpose
Step 1
Router(config)# router ospf process-id
Configures the router to run an OSPF process.
Step 2
Router(config-router)# area area-id filter-list prefix
prefix-list-name in
Configures the router to filter interarea routes into
the specified area.
Step 3
Router(config-router)# ip prefix-list list-name [seq
seq-value] deny | permit network/len [ge ge-value] [le
le-value]
Creates a prefix list with the name specified for the
list-name argument.
To filter interarea routes out of a specified area, use the following commands beginning in router
configuration mode:
Command
Purpose
Step 1
Router(config)# router ospf process-id
Configures the router to run an OSPF process.
Step 2
Router(config-router)# area area-id filter-list prefix
prefix-list-name out
Configures the router to filter interarea routes out
of the specified area.
Step 3
Router(config-router)# ip prefix-list list-name [seq
seq-value] deny | permit network/len [ge ge-value] [le
le-value]
Creates a prefix list with the name specified for the
list-name argument.
Verifying ABR Type 3 LSA Filtering
To verify that the OSPF ABR Type 3 LSA Filtering feature has been configured, use the show ip ospf
EXEC command. The show ip ospf command will show that this feature has been enabled by listing
the area filter as “in” or “out.” The following is sample output from the show ip ospf command:
router# show ip ospf 1
Routing Process "ospf 1" with ID 177.0.0.1
Supports only single TOS(TOS0) routes
Supports opaque LSA
It is an area border router
SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
Number of external LSA 0. Checksum Sum 0x0
Number of opaque AS LSA 0. Checksum Sum 0x0
Number of DCbitless external and opaque AS LSA 0
Number of DoNotAge external and opaque AS LSA 0
Number of areas in this router is 2. 2 normal 0 stub 0 nssa
External flood list length 0
Area BACKBONE(0)
Number of interfaces in this area is 2
Area has no authentication
SPF algorithm executed 6 times
Area ranges are
39.0.0.0/8 Passive Advertise
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Monitoring and Maintaining ABR Type 3 LSA Filtering
===> Area-filter AREA_0_IN in
===> Area-filter AREA_0_OUT out
Number of LSA 5. Checksum Sum 0x29450
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Area 1
Number of interfaces in this area is 1
Area has no authentication
SPF algorithm executed 4 times
Area ranges are
===> Area-filter AREA_1_IN in
===> Area-filter AREA_1_OUT out
Number of LSA 6. Checksum Sum 0x30100
Number of opaque link LSA 0. Checksum Sum 0x0
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
Monitoring and Maintaining ABR Type 3 LSA Filtering
Command
Purpose
Router# show ip prefix-list
Displays information about a prefix list or prefix
list entries.
Configuration Examples
The following configuration example output shows interarea filtering that is applied to both incoming
and outgoing routes:
Router(config)# router ospf 1
log-adjacency-changes
area 1 filter-list prefix AREA_1_OUT out
area 3 filter-list prefix AREA_3_IN in
network 11.0.0.0 0.255.255.255 area 3
network 20.1.1.0 0.0.0.255 area 0
network 55.0.0.0 0.255.255.255 area 1
!
ip prefix-list AREA_1_OUT seq 10 permit 35.25.0.0/8 ge 16
ip prefix-list AREA_1_OUT seq 20 permit 192.20.20.0/24
!
ip prefix-list AREA_3_IN seq 10 permit 172.17.0.0/16
!
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Command Reference
Command Reference
This section documents the area filter-list command that configures the OSPF ABR Type 3 LSA
Filtering feature. All other commands used with this feature are documented in the Cisco IOS
Release 12.0 command reference publications.
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area filter-list
area filter-list
To filter prefixes advertised in type 3 link-state advertisements (LSAs) between Open Shortest Path
First (OSPF) areas of an area border router (ABR), use the area filter-list command. To change or
cancel the filter, use the no form of this command.
area {area-id} filter-list prefix {prefix-list-name in | out}
no area {area-id} filter-list prefix {prefix-list-name in | out}
Syntax Description
area-id
Identifier of the area for which filtering is configured. The identifier can be
specified as either a decimal value or an IP address.
prefix
Indicates that a prefix list is used.
prefix-list-name
Name of a prefix list.
in
Prefix-list applied to prefixes advertised to the specified area from other areas
out
Prefix-list applied to prefixes advertised out of the specified area to other
areas
Defaults
This command has no default behavior.
Command Modes
Router configuration
Command History
Release
Modification
12.0(15)S
This command was introduced.
Usage Guidelines
With this feature enabled in the “in” direction, all type 3 LSAs originated by the ABR to this area, based
on information from all other areas, are filtered by the prefix-list. Type 3 LSAs that were originated as
a result of the area-range command in another area are treated like any other type 3 LSA that was
originated individually. Any prefix that does not match an entry in the prefix list is implicitly denied.
With this feature enabled in the “out” direction, all type 3 LSAs advertised by the ABR, based on
information from this area to all other areas, are filtered by the prefix list. If the area-range command
has been configured for this area, type 3 LSAs that correspond to the area-range are sent to all other
areas, only if there is at least one prefix in the area range that matches an entry in the prefix list.
If all specific prefixes are denied by the prefix list, type 3 LSAs that correspond to the area-range
command will not be sent to any other area. Prefixes that are not permitted by the prefix list are
implicitly denied.
Examples
The following example filters prefixes that are sent from all other areas to area 1:
area 1 filter-list prefix-list AREA_1 in
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area filter-list
The following example filters prefixes that are sent from area 2 to all other areas:
area 2 filter-list prefix-list AREA_2 out
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area filter-list
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